To reduce costs to government, business and householders.
To speed the adoption of cost effective renewable energy solutions.
Whenever subsidies are paid they distort the market place. Organisations and people compete for their slice of the subsidy cake. e.g. you put a solar panel on your roof, the builder charges more because he knows you will get a subsidy and the insurance company charges more for the same reason.
It is possible to produce a renewable heating device that is financially viable without any subsidy. This is the device that should naturally be acepted by the market place, however the proposed RHI initiative will distort the market place by subsidising less cost effective technologies.